Mr Wilson , On Radio-active Rain. 
429 
Glass windows in the front and back of the apparatus enabled 
the position of the gold leaf to be read by a microscope of which 
the eye-piece was provided with a micrometer scale. The time 
taken by the gold leaf to travel over an exact number of scale 
divisions was observed, under normal conditions and when the 
vessel in which the rain had been evaporated was placed inverted 
over the ionisation apparatus as shown in the figure. With this 
apparatus the movement of the gold leaf due to the spontaneous 
ionisation was at the rate of about 11 divisions per hour. 
In the experiments made at Cambridge the apparatus used for 
detecting radio-activity differed from that just described, mainly 
in being inverted, the bottom being closed by a sheet of gold leaf. 
The rain was evaporated in a platinum bowl, which was cooled 
by floating on water and then placed below the ionisation appa- 
ratus. The bowl was a shallow one, wider than the ionisation 
apparatus, so that the surface on which the rain had been dried 
could be brought close up to the gold leaf bottom of the detecting 
apparatus. 
The general result of these experiments has been the same 
at both places. In all cases in which a convenient quantity of 
